Goodhart's Law

Definition

"When a metric becomes a target, it ceases to be a good measure"

Examples

YouTube determined that empirically people watched longer video for a longer time. No surprise there, but it meant that longer videos scored high on the metrics that YouTube tracked, which means that they got higher page rankings by "The Algorithm". So people made longer videos in order to get the higher rankings, and they put all the important stuff at the very end, forcing people to watch all the way through to the end. That adaption by the creators caused the ranks of longer videos to go down.